Conventionally, fastening by bolts and nuts is widely known in mechanical fastening places such as fastening of a number plate to a bumper of automobiles, fastening of a pole base plate to anchor nuts or anchor bolts, fastening in construction sites of iron towers or buildings and fastening parts to railroads or some structures. According to screw up fastenings, it is easy to increase or decrease the fastening rigidity by adjusting fastening torque and it is also possible to readjust the fastening rigidity after the fastening process has been completed if necessary. General tools such as spanners may be utilized to screw up or release bolts and nuts for transmitting torque, and large fastening rigidity can be easily obtained by applying relatively small torque. On the fastened places, various proposed unfastening means may be applied to keep the fastened condition long.
But recently, crimes such as breaking fastening places of number plates, iron towers for power transmission lines and connecting places of rail roads are committed by somebody unknown. Fastening places using bolts and nuts are possible to keep stable and long lasting fastened state by applying appropriate unfastening prevention means. But it is not possible to prevent some body with malicious intent break and loosen fastening means using general tools.
The present applicant has proposed unfastening prevention devices each of which is covered on a fastened place of bolt and nut and is not released by general tools in patent application 2000-148383 (patent publication KOKAI 2001-330020) and patent application 2000-218455 (patent publication KOKAI 2002-39146). In the former patent application 2000-148383, a cylindrical cover member is fixed on a shank of a bolt by a pin. In the patent application 2000-218455, a cover member is fixed on a distal bolt shank projecting from a nut.
But according to the patent application 2000-148383, the pin projects out of the cover member. According to the patent application 2000-218455, it is only applicable at distal end of the bolt shank, and it is not possible to prevent unfastening at head of the bolt.